Hybrid Scavengeless Development (HSD) is a process for electrophotographic imaging and printing apparatuses designed to prevent scavenging of toner from the photoreceptor of the imaging device by subsequent development stations.
In general, the process of electrophotographic printing includes charging a photoconductive member to a substantially uniform potential to sensitize the surface. The charged photoconductive surface is exposed to a light image from either a scanning laser beam, an LED source, or an original document being reproduced. This records an electrostatic latent image on the photoconductive surface. After the electrostatic latent image is recorded on the photoconductive surface, the latent image is developed. Two-component and single-component developer materials are commonly used for development. A typical two-component developer comprises magnetic carrier granules having toner particles adhering triboelectrically thereto. A single-component developer material typically comprises toner particles. Toner particles are attracted to the latent image through electrostatic fields that impart forces to charged toner particles, forming a toner image on the photoconductive surface. The toner image is subsequently transferred to a final substrate such as paper. Finally, the toner powder image is heated to permanently fuse it to the final substrate.
The electrophotographic marking process discussed above can be modified to produce color images. One color electrophotographic marking process, called image-on-image (IOI) processing, superimposes toner powder images of different color toners onto the photoreceptor prior to the transfer of the composite toner powder image onto the substrate. While the IOI process provides certain benefits, such as a compact architecture, there are several challenges to its successful implementation. For instance, the viability of printing system concepts such as IOI processing requires development systems that do not interact with a previously toned image. Since several known development systems, such as conventional magnetic brush development and jumping single-component development, interact with the image on the receiver, a previously toned image will be scavenged by subsequent development if interacting development systems are used. Thus, for the IOI process, there is a need for scavengeless or non-interactive development systems. For a thorough description of scavengeless development see U.S. Pat. No. 5,031,570, hereby incorporated by reference in its entirety.
Hybrid Scavengeless Development technology deposits toner via a conventional magnetic brush onto the surface of a donor roll and a plurality of electrode wires are closely spaced from the toned donor roll in the development zone to the photoreceptor. An AC voltage is applied to the electrode wires to generate a toner cloud in the development zone. This is accomplished as a result of the toner layer on the donor roll being disturbed by electric fields from the wire or set of wires, which produce and sustain an agitated cloud of toner particles in the development nip. Toner from the cloud is then developed onto the nearby photoreceptor by fields created by a latent image.
A problem inherent to such developer systems using wires is vibration of the wires with respect to the donor roll and photoreceptor surfaces. This wire vibration manifests itself as a density variation of toner on the photoreceptor, referred to as “banding.” This banding occurs at a frequency corresponding to the wire vibration frequency. Banding is highly undesirable, as it results in objectionable image quality defects.
The banding toner density variations and the wire vibrations that cause them are lumped together into a problem with the generic name of “strobing.” More specifically, “fundamental strobing” is the term used to describe the vibration and print defect associated with the fundamental mode of vibration of the electrode wire. The frequency of the fundamental mode of vibration is given by the expression
                              w          f                =                              T                          4              *              ρ              *              A              *                              L                2                                                                        Equation        ⁢                                  ⁢        1            wherein T is the wire tension, ρ is the wire density, A is the wire cross section, and L is the length of the wire. One way to minimize strobing is to make the frequency of the fundamental mode as high as possible, because banding at higher frequencies becomes progressively less visible to the naked human eye. Therefore, the tension T is set as high as possible constrained by wire breakage, the limit imposed by the material yield strength. So with a factor of safety α≦1, T can be set to αSyA, where Sy is the yield strength. In this case, the frequency of the fundamental mode can be expressed as,
                              W          f                =                              1                          2              ⁢              L                                ⁢                                                    α                ⁢                                                                  ⁢                                  S                  y                                            ρ                                                          Equation        ⁢                                  ⁢        2            Thus, for a given material, factor of safety, and process width, L, the maximum fundamental mode is proportional to the yield strength divided by the density.
Conventional Hybrid Scavengeless Development electrode wires are often made of stainless steel. For example, the electrode wires are commonly made of 304v stainless steel. Such conventional steel electrode wires exhibit a maximum fundamental resonance frequency in the range of approximately 550 Hz at the required length. This frequency results from steel having a tensile strength of 700 MPa and a density of 7.8 g/cm3. Fundamental strobing is unfortunately visible to the naked human eye at this frequency. The fundamental vibration frequency cannot be further increased while using conventional steel electrodes because the innate physical properties of steel as a material are the limiting factor.
Therefore, there remains in the art a need for an improved HSD system that alleviates fundamental strobing.